Cellular telephone networks are being developed widely at the present time.
One reason for this upsurge is, in addition to the access for subscribers to this type of network, access in a substantially transparent way for subscribers of the public switched telephone network, and vice versa.
In contrast to the public switched telephone network, a cellular telephone network consists, as represented in FIG. 1a, of at least one radio transmitter/receiver RTR per mobile subscriber, linked by radio with a plurality of fixed radio terminals FRT.sub.1 to FRT.sub.3, interconnected in a tree structure by terminal connecting units TCU to a mobile subscriber authentication center PAC, the whole constituting the cellular telephone network proper.
The interconnection between the telephone handset STH of a subscriber of the public switched telephone network PSTN is provided by a unit for access to the mobile telephone network (PAU).
In present-day cellular telephone networks, such as the POINTEL cellular telephone network in Europe, a procedure or protocol for routing incoming calls onto the cellular telephone network, these calls originating from the public switched telephone network, performs specific operations, made necessary by reason of the configuration of the cellular telephone network itself.
In the first place, the abovementioned protocol conventionally requires a procedure for locating each mobile subscriber within the mobile telephone network.
It will be recalled, in connection with FIG. 1b, that such a procedure successively implies:
the sending by the mobile subscriber of a locating request 1; PA1 the forwarding 2 of this request to the mobile subscriber authentication center PAC, via the FRT, the TCU concerned and a dedicated transmission line network; PA1 the verification of access rights, then the memory storage, by the PAC, of the network address or identity of the FRT terminal identified as locating terminal, so as to allow the latter to route any subsequent incoming call to this mobile subscriber. PA1 1. The RTR is assumed to be located in the vicinity of the locating terminal FRT.sub.1. For example, the caller from the public switched telephone network dials the call number of the mobile called party, which is forwarded to the PAU constituting, within the PAU, a first half-call. PA1 2. The PAU, having available parameters for locating the called mobile subscriber, indicates to the PAC that a call request exists intended for this mobile subscriber, and communicates a number for calling the mobile subscriber to the mobile subscriber authentication center PAC. PA1 3. The PAC calls the TCU with the locating terminal FRT.sub.1 attached, indicating the call number. PA1 4. The TCU indicates to the locating terminal FRT.sub.1 that the transmitter/receiver RTR with the call number corresponding to this mobile subscriber is being sought. PA1 5. The locating terminal FRT.sub.1 embarks on a polling procedure in order, by sending call messages, to search for the transmitter/receiver RTR of this mobile subscriber on the radio link. After a set time, of the order of a few seconds, in the absence of a response from the mobile subscriber in question, the locating terminal FRT.sub.1 sends its connecting TCU a negative report. The procedure of calling the locating terminal constitutes a second half-call. PA1 6 and 6'. The connecting TCU sends each of the daughter terminals FRT.sub.2, FRT.sub.3, other than the locating terminal FRT.sub.1, simultaneously, a request to search for the transmitter/receiver RTR of the mobile subscriber at the corresponding call number. PA1 7. Each daughter terminal embarks on a polling procedure similar to that of stage 5 previously conducted by the locating terminal, each call constituting a second half-call. PA1 7'. When a daughter terminal FRT.sub.2, FRT.sub.3, has found the transmitter/receiver of the mobile subscriber in question, that is to say when a message acknowledging reception of the last call generated by the terminal is received back, this terminal rings the transmitter/receiver RTR so as to call the user mobile subscriber. PA1 8. When this user mobile subscriber picks up his transmitter/receiver RTR, the abovementioned daughter terminal, FRT.sub.2 for example, calls the PAU back via the connecting TCU and a dedicated telecommunications network, the PAU then joining the two half-calls together. PA1 implementing a procedure, from the mobile subscriber transmitter/receiver, for locating/authenticating this transmitter/receiver, via the mobile telephone network and the mobile subscriber authentication center, and, upon the mobile subscriber transmitter/receiver being located and authenticated by a fixed radio locating terminal, PA1 forwarding, from the mobile subscriber authentication center to the fixed radio locating terminal, the personal communication service subscription data, identification number and confidential code of the mobile subscriber transmitter/receiver, furthermore, within the fixed radio terminal, attributing to this mobile subscriber transmitter/receiver a free direct inward dialling number, PA1 forwarding, from the locating terminal to the service access authentication center, via the voice locating server, the direct inward dialling number attributed, the identification number and the confidential code, the address of the fixed radio locating terminal associated with the mobile subscriber transmitter/receiver being stored in memory within the service access authentication center, and upon an incoming call for the telephone call number of this mobile subscriber transmitter/receiver generated by this switched telephone handset to the service control point, PA1 forwarding, from the service control point to the service access authentication center, an interrogation message including at least the telephone number of the mobile subscriber transmitter/receiver; PA1 upon reception of this interrogation message forwarding, to the fixed locating terminal, from the service access authentication center, via the service control point, a call message containing at least the direct inward dialling number attributed; PA1 from the fixed radio locating terminal, implementing a procedure of calling the mobile subscriber transmitter/receiver, and, upon the mobile subscriber responding PA1 establishing communication with the telephone handset.
When such an incoming call appears, it is, by reference to FIG. 1a, handled by the unit for access to the mobile telephone network PAU, on the basis of a database giving information on all the daughter FRT terminals which are interconnected to the same terminal connecting unit, TCU.
Such a protocol for handling an incoming call is summarized below in connection with FIG. 1a and entails, by virtue of the sending of requests or successive messages where the dashed lines represent the transmission of messages by voice and the solid lines the transmission of messages by dedicated link, the following stages:
The abovementioned protocol exhibits drawbacks, relating essentially to the structure of the mobile radio telephone network itself. Among these drawbacks, it may firstly be mentioned that the daughter terminals are handled manually when one or more FRT terminals are added.
It may also be mentioned that the locating and the incoming calls in the mobile telephone network are handled by specific equipment units of the network.
Finally, mention may be made of the fact that the solutions employed in the mobile telephone network do not make it possible to avoid the effect known as the "trombone" effect, as illustrated in FIG. 1c. If, as represented in the abovementioned FIG. 1c, a mobile subscriber from a town X, Paris, on a visit to a town Y, Strasbourg, is temporarily located, by the previously described protocol, in the town Y, Strasbourg, an incoming call originating from a subscriber of the public switched telephone network, and intended for this mobile subscriber, will necessarily pass through the town X, Paris. This is the case especially for an incoming call originating from a subscriber of the public switched telephone network, located in the town Y, Strasbourg, for which such a call has to pass through the town X, Paris, needlessly using up resources of the switched telephone network. This gives rise to the notion of a "trombone" effect illustrating the outwards and return path of the call between towns Y, Strasbourg, and X, Paris.
Such a routing of the incoming telephone calls furthermore gives rise to increased costs for the users.